This invention relates generally to nuclear power generating stations, and more particularly to a method and apparatus for the generation and maintenance of evidence to assure that qualified equipment used in such stations will operate on demand, to meet the system performance requirements.
Safety system equipment in nuclear power generating stations is required to be capable of performing satisfactorily under defined conditions, including postulated design basis events such as a loss of coolant accident. It is necessary to provide assurance that such equipment meets its performance requirement throughout its installed life. This is accomplished through a disciplined program of design, qualification, production quality control, installation, maintenance, and surveillance testing.
The process of qualification usually also establishes the qualified lifetime of the equipment. That is the period of time, prior to the start of a design basis event, for which the equipment is demonstrated to meet the design requirements for the specified service conditions. At the end of qualified lifetime, such equipment should be capable of performing the safety functions required for the postulated design basis, and post-design basis events. The qualified lifetime of equipment may be extended if it can be shown that the service or environment conditions originally assumed were overly conservative with respect to those that apply at the equipment's location in its installed configuration. Use of extension methods requires documentation of the program to be followed and auditable records of the results, as illustrated in the IEEE Std 323-1974, "IEEE Standard for Qualifying Class IE Equipment for Nuclear Power Generating Stations", which is incorporated herein by reference.
Gamma ray and beta radiation exposure occurring during normal nuclear power plant operations, or occurring as a result of an accident, can alter the performance of certain equipment such as motors, valve and motor operators, transmitters, indicators, instruments, organic fluids, seals, and electrical insulation. For this reason, equipment qualification includes appropriate exposure to radiation as part of the test procedure. The total radiation dose to which the equipment is exposed in qualification testing depends upon the expected normal radiation environment and the length of time following an accident that the equipment is required to operate.
Nuclear power plant personnel will require a knowledge of the total integrated dose of radiation that a piece of equipment has received if it is desired to extend the equipment qualification beyond the present qualified life, or if it is desired to requalify equipment following an accident. As is known, the term "total integrated dose of radiation" refers to an accumulated amount of each type of radiation which is received by the equipment over a period of time, and which is a combination of normal and accident doses. Depending upon the particular form of accident which occurs and the operational conditions of the plant, such normal and accident doses will include not only gamma and beta radiation, but also other forms of radiation such as neutron, electron, X-ray, etc. In general, the radiation doses used for equipment qualification are sufficiently conservative that, if the actual integrated doses are known, the equipment could continue in service without exceeding the existing qualification envelopes. This is particularly true for an accident where the distribution of radiation levels in the nuclear power plant will vary widely, and be much lower than that assumed for qualification, due to radioactive decay during the release, the physical distribution of radioactive material, and the presence of local shielding in the form of structure, other equipment, floors, and walls.
Previously, nuclear power plant personnel would have very few options with regard to thermal and radiation aging of important equipment. Qualified equipment could be replaced when it reached the end of its qualified lifetime in calendar years without regard to the actual environmental aging; or, nuclear power plant life extension decisions and decisions regarding qualified equipment could be made based upon a projection of very limited active temperature and radiation level measurements. Without measured data, the affected equipment could either be replaced, refurbished, or disassembled, inspected, and reassembled. Major problems with such approaches, however, exist in the time and cost for replacement or refurbishment, as well as considerations affecting qualified equipment vendors and qualified repair parts. Therefore, what is needed is a passive system that provides integrated radiation dose and thermal history throughout normal plant operation, through the duration of a nuclear power plant accident, and beyond.